Systems for the communication of signals having low frequencies, such as music, wherein the signals are modulated on a carrier present a problem in that the spectral lines representing the signal are closely spaced to the carrier spectral line. Synchronous detection of the signals is difficult because of the difficulty in extracting a carrier from signal spectra lying close to the carrier. A pilot tone having a frequency proportional to the carrier, but lying outside the signal transmission band may be employed for synthesizing a carrier. However, atmospheric perturbations introduce frequency and phase dispersion between the pilot tone and the carrier thereby preventing the synthesis of a carrier which is free of unwanted phase shift, this phase shift reducing the precision of the detection.